It is possible to measure a temperature using platinum resistance thermometer elements, more precisely than using other temperature sensors such as thermocouples. Thus, platinum resistance thermometer elements are widely used in the case where precise temperature measurement is required.
Examples of commonly used platinum resistance thermometer elements include an element in which a coiled platinum resistance wire is accommodated in a ceramic insulator (FIG. 1 of JP 557-126035U) and an element in which a platinum resistance wire wound around a glass bobbin is coated by glass (FIG. 4 of JP 544-29830B), and further include an element in which a platinum thin film in the form of a meandering wire is formed on a thin film made of ceramic or the like to form a platinum resistance wire, and the surface thereof is insulated by coating (JP 2003-179276A). Since the electrical resistance of a platinum resistance wire changes in accordance with the temperature, temperature measurement using a platinum resistance thermometer element is performed by measuring the electrical resistance of a platinum resistance wire, and converting the obtained value into the temperature.
In a platinum resistance thermometer element, a platinum resistance wire is protected by a member made of a fragile material such as ceramic or glass as described above. Thus, an external force such as vibrations may damage the protective member, resulting in a break or short-circuit in the platinum resistance wire.
Thus, it is uncommon for a platinum resistance thermometer element to be used alone as a temperature sensor, and the platinum resistance thermometer element is usually used as a temperature sensor in the form where the resistance thermometer element is accommodated in an outer frame made of metal or the like, for example, as shown in Patent Document 1.
FIG. 4 is a cross-sectional view showing a structure widely used in a temperature sensor using a platinum resistance thermometer element. A temperature sensor 10 is configured such that a platinum resistance thermometer element 4 is fixed via an insulating filler 90 in a metal outer frame 70. Regardless of the type of the platinum resistance thermometer element 4, platinum lead wires 6 connected to a platinum resistance wire are usually exposed from a platinum resistance thermometer element main body 5 containing the platinum resistance wire, and ends of the platinum lead wires 6 and ends 160 of three electric conductive wires 140 of a three-wire cable 30 for measuring the electrical resistance of the platinum resistance wire are connected by welding.
Although FIG. 4 is a cross-sectional view, the platinum resistance thermometer element 4 and a cover 130 of the cable 30 are shown in the external shapes thereof for facilitating visualization. As is known, types of the temperature measurement using a platinum resistance thermometer element include two-wire measurement, three-wire measurement, and four-wire measurement, among which three-wire measurement is most widely used in the industrial field. FIG. 4 shows a case of three-wire measurement in which the three electric conductive wires 140 of the cable 30 are connected to the platinum lead wires 6 exposed from the platinum resistance thermometer element main body 5.
In the description below, a “temperature sensor for high temperature” refers to a temperature sensor for high temperature using a platinum resistance thermometer element.
Regarding the insulating filler 90, since a ceramic adhesive has excellent thermal resistance and the same is applied to a ceramic powder, a ceramic adhesive or a ceramic powder tightly filled at a high density is used as the insulating filler 90 in a temperature sensor for high temperature used in a high temperature range over 200° C. A ceramic powder is tightly filled at a high density in order to achieve good thermal conduction so as not to impair the response speed in the temperature measurement. Furthermore, the cable 30, in the case of the temperature sensor for high temperature, is a so-called MI cable in which the three electric conductive wires 140 are accommodated via a ceramic powder in the cover 130 configured by a metal sheath, in order to achieve good thermal resistance. Note that the maximum temperature of a platinum resistance thermometer element as defined in IEC Standard and JIS Standard is 850° C.
Furthermore, a “ceramic adhesive” refers to a ceramic-based adhesive mainly made of ceramic.
When a ceramic powder tightly filled at a high density is used as the insulating filler 90, the ceramic powder may absorb moisture and deteriorate in the insulation resistance, causing errors in the temperature measurement, and, thus, the metal outer frame 70 is usually provided with a lid, and the lid is welded or brazed to the outer frame 70 and the metal sheath of the cable 30, so that the ceramic powder accommodated inside is shielded from the outside air, thereby preventing errors in the temperature measurement.